Set func as the callback that will be called when a new message needs to be sent to the client.
user_data is passed to func and notify is called when user_data is no longer in use.
By default, the client will send the messages on the Connection that was configured with attach was called.
It is only allowed to set either a `send_func` or a `send_messages_func` but not both at the same time.
| this |
a Client |
| func | |
| notify |
called when |
| user_data |
user data passed to |